A standard plug-in proximity switch has a tubularly cylindrical housing containing a printed-circuit board carrying the active and passive elements of the switch. One end of the housing is formed as a plug that can be fitted to a socket to install the unit. The circuit, which is invariably mounted on a printed-circuit board, must be fitted with a plug that can be inserted into a socket for connection to the equipment or installation it is used in. As a rule the board is contained in a housing and is largely surrounded by insulating potting compound. A plug assembly with projecting connection pins is mounted on one end of the housing and is connected inside the housing with the circuit board.
Such a proximity switch is described in German utility model 8,709,199. In it the connection between the circuit board and the plug elements is made by conductive leads whose inner ends are soldered to contact points on the circuit board which is then fitted into a tubular housing or initiator tube. The tube is then stood on end and at least partially filled with the insulating potting compound, which hardens. Subsequently the plug elements are soldered to outer ends of the leads and the plug unit is fixed in the open end of the tubular housing.
As also described in German patent 3,840,678 it is known to provide a fill passage or hole in the plug so that once the entire unit is assembled, it can be completely filled with potting compound. This makes the assembly very rugged.
The main disadvantage of this arrangement is that it is fairly bulky and expensive to make. The leads must be accommodated between the plug and the circuit board, and connecting both ends of the leads is fairly time consuming and, in fact, capable of producing a cross-wiring error.